1. Field of the Invention
The invention relates to the field of measuring technology. The invention relates to the field of detecting defects of steel strip by utilizing a magnetic defect detector. The apparatus of the invention is especially useful for inspection of a steel strip coated with tin and formed into a can for storing foods by a deep drawing process.
2. Description of the Related Art
In the process of making steel strip, especially tin plate strip, it is required to detect the defects on or in the strip. A recent technology in making cans for storing food is the process wherein the tin plate is deep drawn to form a part of the can corresponding to the body and the bottom end of a three piece can, into one piece, in one step. This is a great advance in can making technology. However the technology puts a heavy burden on the quality of the tin plate used for can making. In deep drawing, the severe working may cause breakage of the tin plate on the inner, or on the outer surface of the steel plate. Therefore, minor defects which may stand in the other working can be the ones of which detection is required for the tin plate utilized in the deep drawing. This improvement of the measuring technology is also beneficial to the steel plate used for general purposes.
There are related arts in this field of technology wherein detection is performed of the defects of the steel strip such as inner defects or bubbles, or surface defects, when the steel plate is stationary or running. Japanese Utility Model laid open 107849/88 dicloses such an apparatus for the detection of the defects.
FIGS. 3A and 3B are partially cutaway sectional views of a related art, with FIG. 3A being a side view and FIG. 3B being a front view of the related art. FIGS. 3A and 3B show an apparatus which continuously detects defects of a running steel strip. In these figures, 1 denotes a hollow cylinder made of a nonmagnetic material. The shaft 2 is placed at the center of the hollow cylinder 1. The shaft 2 is fixed to a stationary body, not shown. A pair of bearings 3a and 3b are attached between the shaft and the hollow cylinder at both ends of the hollow cylinder. Accordingly, the hollow cylinder can be freely rotated around the shaft. Inside of the hollow cylinder, the magnetizing yoke 4 is fixed to the shaft by the engagement of the supporting member 5, the free ends of the yoke being placed in the vicinity of the inner surface of the cylinder. The yoke is equipped with magnetizing coil 6. Between both free ends of the yoke the magnetic detection elements 7 is placed and fixed to the shaft. The cable 8 which supplies the magnetizing electric current, and the cable 9 which takes out the signal from the magnetic detection elements are led to the outside of the hollow cylinder via the shaft. Accordingly, the magnetizing yoke and the magnetic detection elements are fixed and the hollow cylinder rotates around the yoke and the element, with little clearance between the inner surface of the cylinder and the yoke or the elements. In this configuration of the cylinder, when the strip 10 is running in the direction denoted as A, it is pressed to the cylinder by a certain force, and the cylinder rotates in that direction. When magnetizing electric current is supplied, a closed magnetic circuit is formed between the yoke and the running steel strip 10. When a defect is in existence in or on the strip, the magnetic circuit is disturbed, and a magnetic flux leakage is generated which is detected by the magnetic detection elements as a defect signal.
The detected defect signal corresponds to the magnitude of the defect at the inside or on the surface of the strip, and the the magnitude of the defect can be classified by the level of the signal.
The level of the defect signal varies with the magnetic circuit condition which is determined by the magnetizer composed of the yoke and the magnetizing coil, and the strip, and with "lift off", or the distance between the magnetic detection element and the strip, denoted as "d".
FIG. 4 is a graph showing the relationship between "d" and the signal level of the defect signal. The abscissa denotes "d" and the ordinate denotes the signal level. As shown in FIG. 4, the larger the distance between the magnetic detection elements and the strip, the smaller the signal level. To evade the problem, the "d" is maintained constant by using hollow cylinder having the thickness of "t" as shown in FIGS. 3A and 3B. The hollow cylinder is made of a nonmagnetic material since if it has magnetism, the formation of the magnetic circuit is not possible.
The magnetic circuit is stabilized when the magnetic gap between the free ends of the yoke and the strip gets smaller or the "t" gets smaller. When the "t" gets smaller, the "d" gets smaller, the signal level gets larger, and signal noise level ratio gets larger which leads to the enhancement of the accuracy of the detection. Accordingly it is desirable to make the thickness of the hollow cylinder as thin as possible.
To carry out the detection when the strip is running, the hollow cylinder and the strip should be constantly contacted. As the result of the application of the tension on the strip to make the contact with the cylinder, the downward force due to the tension and the weight of the strip is applied on the hollow cylinder which deforms the cylinder or may break it. When the cylinder is deformed, the distance between the magnetic detection element and the strip "d" can not be maintained constant. Therefore the thickness of the hollow cylinder "t" can not be made thinner than a predetermined restriction to maintain the circular shape of the hollow cylinder as long as possible.
Accordingly the apparatus of the related art shown in FIGS. 3A and 3B is incapable of performing the detection of minor defects, of the strip running in the production line or in the processing line because the thickness of the cylinder can not be as this as the desired quality requirement of the strip.